Measurement of voltage standing wave ratio of antenna system

ABSTRACT

The present invention provides methods and apparatus to detect the presence of interferers in a wideband digital VSWR measurement signal. Interferers cause power differences over different intervals of time and frequency, and may be detected by comparing the measured signals in both time and frequency domains with the original signal. Frequency components in the measured signals may be discarded if the interference is deemed too large. The remaining frequency components can then be used to compute the VSWR.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.13/424,101, filed Mar. 19, 2012, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present invention relates generally to measurement of the VoltageStanding Wave Ratio (VSWR) of antenna systems and, more particularly, tomeasurement techniques to reduce the impact of interfering signals inthe calculation of the VSWR.

BACKGROUND

Traditionally, the Voltage Standing Wave Ratio (VSWR) of antenna systemsis measured using filtered or narrow-band power measurements. Bymeasuring the power of the forward and reflected signals (P_(fwd) andP_(ref)l) simultaneously, the scalar return loss (RL) and reflectionco-efficient (Γ) can be calculated.

Typical VSWR measurement systems consist of a directional coupler, andanalog power detectors to measure the voltage of the forward signal,denoted V_(fwd), and the voltage of the reflected signal, denotedV_(refl). A mixer circuit and low pass filter are used to measure theVSWR over a specific frequency and bandwidth. An analog to digitalconverter (ADC) quantizes the power detector value. High values ofV_(refl) relative to V_(fwd) indicate potential transmission line andantenna faults. Both V_(refl) and V_(fwd) will be affected if there isan interference signal coupled into the measurement within or evenadjacent to the measurement bandwidth.

The current analog VSWR (AVSWR) and narrowband digital VSWR (DVSWR)measurement systems are unable to detect or account for interferers asthey have no knowledge of the dynamic interferer. False alarms can beraised if the interference is large enough to raise the V_(refl) suchthat it looks like a cable fault. False alarms may cause outages in thenetwork if the transmission signal is turned off by fault managementsoftware to prevent damage to the radio in the event of a cable fault.False alarms also may cause unnecessary maintenance on the radio andantenna system. Therefore, special tests are used to verify thesensitivity of the AVSWR measurement to adjacent interferers.

Another drawback is that the AVSWR measurement can only be performedover a small bandwidth determined by the configured frequency of themixer and low pass filter. The AVSWR method cannot measure the V_(fwd)or V_(refl) separately. Any voltage induced on the antenna system due toan interferer will be coupled into both the V_(fwd) and V_(refl)signals, and will impact the accuracy of the measurement. The level ofcoupling is based on the S-parameters of the directional coupler.

The AVSWR measurement method also does not compare the V_(fwd) orV_(refl) with the original transmitted signal, denoted V_(ref), and thusdoes not determine if there is any interference that impacts the overallaccuracy of the measurement.

SUMMARY

The present invention provides methods and apparatus to detect thepresence of interferers in a wideband digital VSWR measurement signal.Interferers cause power differences over different intervals of time andfrequency, and may be detected by comparing the measured signals in bothtime and frequency domains with the original signal. Frequencycomponents in the measured signals may be discarded if the interferenceis deemed too large. The remaining frequency components can then be usedto compute the VSWR.

Exemplary embodiments of the invention comprise methods of measuring theVSWR in a wideband communication system. In one exemplary embodiment,time-domain measurement signals for a forward wave and a reflected waveare converted to a frequency domain. Reflection coefficients are thencomputed for a plurality of frequency components in the measurementsignals. The reflection coefficients are used to compute interferencedetection metrics for one or more of the frequency components in themeasurement signals. Frequency components in the forward wave, thereflected wave, or both, may be discarded based on the interferencedetection metrics, and the the remaining frequency components in theforward and reflected waves are used to compute the VSWR.

Other embodiments of the invention comprise a wireless terminalconfigured to compute a VSWR. The wireless terminal comprises adirectional coupler connected between a transmitter and an antenna, areceiver circuit to generate time domain measurement signals for forwardand reflected waves, and a processing circuit for computing the VSWR.The processing circuit is configured to convert time-domain measurementsignals for the forward wave and the reflected wave to a frequencydomain; compute reflection coefficients for a plurality of frequencycomponents in the measurement signals as a function of the reflectioncoefficients, compute interference detection metrics for one or more ofthe frequency components in the measurement signals, discard one or morefrequency components in the forward wave or the reflected wave, or both,based on the interference detection metrics, and compute a VSWR for theforward wave and the reflected wave based on the remaining frequenciesin the forward and reflected waves.

The present invention enables the detection of an interfering signal inmeasurement signals used to compute the VSWR. As a result, elements inthe measurement signals containing interference may be discarded beforethe VSWR is computed. False alarms can be avoided, resulting in fewerservice calls and network outages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless terminal including a measurement circuitfor measuring the voltage standing wave ratio (VSWR) of the antennasystem.

FIG. 2 illustrates exemplary processing for computing the VSWR accordingto one exemplary embodiment.

FIG. 3 illustrates exemplary processing for computing the VSWR accordingto a second exemplary embodiment.

FIG. 4 illustrates an exemplary method of computing a VSWR for anantenna system.

FIG. 5 is an exemplary graph of the measured coupling factor showing aninterfering signal.

FIG. 6 is an exemplary graph of the forward coupling factor errorshowing an interfering signal.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary wireless terminal 10 including ameasurement circuit 20 for determining a voltage standing wave ratio(VSWR). A signal source 12 generates a digital signal for transmissionover a wireless channel. The digital signal is applied to the input of atransmitter 14. After conversion to analog form, the transmitter 14upconverts, filters, and amplifies the signal. The output of thetransmitter 14 is coupled via a transmission cable 16 to a transmitantenna 18. A measurement circuit 20 is coupled to the transmissioncable 18 between the transmitter 14 and antenna 18 to measure thevoltage standing wave ratio (VSWR) of the antenna system. As will behereinafter described in greater detail, the measurement circuit 20 isconfigured to detect interferers in the signals on which measurementsare made, and to remove frequency components of the measurement signalscontaining excessive interference.

The measurement circuit 20 includes a directional coupler 22, forwardreceiver 24, a reverse receiver 26, and a signal processing circuit 28.The directional coupler 22 generates scaled versions of the transmittedsignal, i.e the forward signal, and reflected signal. An interferingsignal may be coupled by the directional coupler 22 into both theforward and reflected signals. The forward receiver 24 receives thescaled version of the transmit signal, referred to as the forwardmeasurement signal (sigFwd). The forward receiver 24 demodulates anddigitizes the forward measurement signal. The reverse receiver 26receives a scaled version of the reflected signal, referred to as thereflected measurement signal (sigRefl). The reverse receiver 26demodulates and digitizes the reflected measurement signal. Thedigitized measurement signals are input to the signal processing circuit28. Additionally, the original signal is applied to the signalprocessing circuit 28 and used as a reference signal (sigRef).

The signal processing circuit 28 has two main functions. First, thesignal processing circuit 28 detects the presence of interfering signalswithin or adjacent to the measurement bandwidth. As described in greaterdetail below, interfering signals may be detected by comparing themeasurement signals with the original transmitted or reference signal inboth the time and frequency domain. Second, the signal processingcircuit 28 calculates the VSWR of the antenna system based on themeasurement signals. For purposes of calculating the VSWR, frequencycomponents in the measurement signals corrupted by interference arediscarded.

FIG. 2 illustrates exemplary processing steps 100 performed by thesignal processing circuit 28 in one embodiment. The inputs to the signalprocessing circuit 28 include sigRef, sigFwd, and sigRefL. The signalssigRef, sigFwd, and sigRefL are sampled over a finite period of time.The measurement signals sigFwd and sigRefL may be sampled as RF signalsand converted to the digital domain. The reference signal (sigRef) maycomprise a baseband signal, or may be sampled as an analog RF signal ina manner similar to sigFwd and sigRefL.

The signal processing circuit 28 cross correlates sigFwd with sigRef tofind the peak correlation and determine the sample offset (block 102).Once the sample offset is known, the signal processing circuit 28time-aligns sigFwd and sigRefL with sigRef (block 104). The signalprocessing circuit 28 then performs a Discrete Fourier Transform (DFT)or other transform operation to convert the time-aligned signals(sigRef, sigFwd′, sigRefl′) from the time domain to the frequency domain(block 106). The frequency domain signals (refSigFreq, rawFwdFreq, andrawReflFreq) represent the magnitude and the phase of the signals withrespect to frequency. The signal processing circuit 28 compensatesrawFwdFreq and rawReflFreq for nonlinearities in the analog circuits byapplying calibration factors determined during radio calibrationprocedures (block 108). The signals output from the analog compensationfunction are the measured forward frequency (measFwdFreq) and measuredreflected frequency (measReflFreq).

The signal processing circuit 28 calculates a measured forward couplingfactor, (fwdCfMeas) for each of a plurality of frequency bins (block110). The forward coupling factor for a given frequency bin is equal tothe measured forward frequency divided by the reference signal frequency(measFwdFreq/refSigFreq). FIG. 5 shows a graph of the forward couplingfactor for a white noise reference signal with a GSM interferer. Themeasured forward coupling factor (fwdCfMeas) is compared against theknown coupling factor (fwdCfCal) determined at the time of calibration.The difference between the measured forward coupling factor and thecalibrated coupling factor is computed to obtain an absolute forwardcoupling factor error (absFwdCf Err) (block 112). FIG. 6 shows a graphof the forward coupling factor error for a white noise reference signalwith a GSM interferer.

The forward coupling factor error is used as an interference detectionmetric. A high forward coupling factor error indicates the presence ofan interfering signal. Therefore, the forward coupling factor errorcomputed for each frequency bin can be compared to an error threshold(block 114). If the forward coupling factor error is greater than orequal to the error threshold, it is determined that interference ispresent in the corresponding frequency bin (block 116).

The signal processing circuit 28 also computes a measured reflectioncoefficient (measReflCoeff) for each frequency bin. The measuredreflection coefficient is computed by dividing the measured forwardfrequency by the measured reflected frequency(measFwdFreq/measReflFreq). The measured reflection coefficients forfrequency bins where interference is present are then discarded (block120). The measured reflection coefficient for the remaining frequencybins are used to compute the VSWR (block 122). The VSWR is given by:

${VSWR} = {\frac{V_{\max}}{V_{\min}} = \frac{1 + \rho}{1 - \rho}}$where ρ=|Γ| and Γ is the measured reflection coefficient.

FIG. 3 illustrates exemplary processing steps 150 performed by thesignal processing circuit 28 in another exemplary embodiment. In thisembodiment, the measured reflection coefficient is compared to anexpected reflection coefficient to detect the presence of interferingsignals in the measurement signals. The frequency components containinginterference are discarded and the VSWR is calculated based on theremaining frequency components.

The signal processing circuit 28 cross-correlates the forward signalwith the reference signal to determine a sample offset (block 152). Thesample offset determined by the cross-correlation is used to time alignthe forward signal and reflected signal (block 154). The time-alignedsignals (sigFwd′, sigRefl′) are then converted from the time domain tothe frequency domain by a DFT operation (block 156). Analog compensationis applied to the raw frequency domain signals (rawFwdFreq andrawReflFreq) as previously described to obtain a measured forwardfrequency (measFwdFreq) and measured reflected frequency(measReflFreq)(block 158).

A measured reflection coefficient (measReflCoeff) is computed bydividing the measured forward frequency by the measured reflectedfrequency (block 160). The measured reflected coefficient is input to atrend-fitting algorithm to obtain an expected reflection coefficient(reflCoeffExp)(block 162). The signal processing circuit 28 determines adifference between the measured reflected coefficient and the expectedreflected coefficient (block 164). This difference represents theabsolute reflection coefficient error (absReflCoeffErr). Becauseinterfering signals will result in a detectable error in the reflectioncoefficient over the bandwidth of the measured signals, the reflectioncoefficient error can be used as an interference detection metric.

The reflection coefficient error is compared to an error threshold(block 166). If the reflection coefficient error exceeds the errorthreshold, it is determined that the corresponding frequency componentcontains an interfering signal (block 168). The frequency componentscontaining the interfering signals are discarded (block 170). Themeasured reflection coefficients for the remaining frequency componentsare used to compute the VSWR as previously described (block 172).

FIG. 4 illustrates a general method 200 performed by the signalprocessing circuit 28 to compute the VSWR. The signal processing circuit28 converts time domain and measurement signals to the frequency domain(block 210). As previously noted, the frequency domain representationrepresents the magnitude and phase as a function of frequency. Thebandwidth is divided into a plurality of frequency bins. The signalprocessing circuit 28 computes reflection coefficients for each of thefrequency bins (block 220). The signal processing circuit 28 alsocomputes an interference detection metric for each frequency bin (block230). In the embodiment shown in FIG. 2, the interference detectionmetric comprises the forward coupling factor error. In the embodimentshown in FIG. 3, the interference detection metric comprises thereflection coefficient error. Based on the interference detectionmetric, the signal processing circuit 28 discards frequency componentsin the measurement signals corresponding to frequency bins where theinterfering signal is detected (block 240). The VSWR is then computedbased on the remaining frequency components (block 250).

The techniques described herein enable the detection of an interferingsignal in measurement signals used to compute the VSWR. As a result,frequency components in the measurement signals containing interferencemay be discarded. False alarms can be avoided, resulting in fewerservice calls and network outages. Thus, network operators would see asignificant savings. In cases where the interfering signal ispersistent, the interfering signal can be identified and an appropriateresponse can be sent to the operator (alarms, warnings, or othercorrective action).

Thus, the foregoing description and the accompanying drawings representnon-limiting examples of the methods and apparatus taught herein. Assuch, the present invention is not limited by the foregoing descriptionand accompanying drawings. Instead, the present invention is limitedonly by the following claims and their legal equivalents.

What is claimed is:
 1. A measurement circuit for measuring a voltagestanding wave ratio (VSWR) in an antenna system, the measurement circuitcomprising: a directional coupler connected between a transmitter and anantenna; a receiver circuit to generate measurement signals for forwardand reflected signals in a transmission line of the antenna system; anda processing circuit configured to: compute reflection coefficients fora plurality of frequency bins based on the measurement signals; detectinterference in one or more of the frequency bins by computinginterference detection metrics for one or more frequency bins among saidplurality of frequency bins as a function of the reflectioncoefficients, wherein the interference detection metrics comprises aforward coupling factor error or a reflection coefficient error, whereinit is determined that a corresponding frequency bin contains aninterfering signal if the forward coupling factor error or thereflection coefficient error exceeds an error threshold; discardreflection coefficients for one or more frequency bins in whichinterference has been detected; and compute the VSWR of the antennasystem based on the reflection coefficients for a remaining number offrequency bins.
 2. The measurement circuit of claim 1, wherein theprocessing circuit is configured to control, based on the computed VSWR,transmission of a signal on the transmission line.
 3. The measurementcircuit of claim 2, wherein the processing circuit is configured tocontrol by sending a control signal to refrain from transmitting thetransmission signal on the transmission line.
 4. The measurement circuitof claim 1, wherein the processing circuit is configured to detect atransmission line or antenna fault based on the computed VSWR, and senda control signal for the antenna system responsive to the detection. 5.The measurement circuit of claim 1, wherein: the receiver circuit isconfigured to generate measurement signals by generating time-domainmeasurement signals; and the processing circuit is configured to:compute discrete Fourier transforms of the measurement signals for theforward signal and the reflected signal to convert the measurementsignals to the frequency domain, and compute the reflection coefficientsbased on the frequency-domain measurement signals.
 6. The measurementcircuit of claim 1, wherein the processing circuit is configured todivide a magnitude of the measurement signal for the reflected signal bya corresponding magnitude of the measurement signal for the forwardsignal to compute the reflection coefficients.
 7. The measurementcircuit of claim 1, wherein, to detect interference in one of more ofthe frequency bins, the processing circuit is configured to compare theinterference detection metrics for one or more frequency bins to theerror threshold.
 8. The measurement circuit of claim 1, wherein theprocessing circuit is configured to determine measurement errors foreach of the frequency bins as a function of its corresponding reflectioncoefficient, and wherein the measurement errors comprise interferencedetection metrics for determining interference in one or more of thefrequency bins.
 9. A method of measuring a voltage standing wave ratio(VSWR) of an antenna system, the method comprising: generatingmeasurement signals corresponding respectively to a forward signal and areflected signal in a transmission line of the antenna system; computingreflection coefficients for a plurality of frequency bins based on themeasurement signals; detecting interference in one or more of thefrequency bins by computing interference detection metrics for one ormore frequency bins among said plurality of frequency bins as a functionof the reflection coefficients, the interference detection metricscomprising a forward coupling factor error or a reflection coefficienterror, wherein it is determined that a corresponding frequency bincontains an interfering signal if the forward coupling factor error orthe reflection coefficient error exceeds an error threshold; discardingreflection coefficients for one or more frequency bins in whichinterference has been detected; and computing the VSWR of the antennasystem based on the reflection coefficients for a remaining number offrequency bins.
 10. The method of claim 9, wherein the method isimplemented by a wireless terminal comprising the antenna system, andwherein the wireless terminal computes the VSWR to control a transmitterof the wireless terminal.
 11. The method of claim 10, wherein the methodcomprises controlling, based on the computed VSWR, transmission of asignal on the transmission line, wherein the controlling comprisesrefraining from transmitting the transmission signal on the transmissionline.
 12. The method of claim 10, wherein the method comprises detectinga transmission line or antenna fault based on the computed VSWR, andsending a control signal to refrain from transmitting a transmissionsignal on the transmission line.
 13. The method of claim 9, wherein: thegenerating measurement signals comprises generating time-domainmeasurement signals; the method comprises computing discrete Fouriertransforms of the measurement signals for the forward signal and thereflected signal to convert the measurement signals to the frequencydomain; and computing the reflection coefficients based on thefrequency-domain measurement signals.
 14. The method of claim 9, whereincomputing reflection coefficients for a plurality of frequency binsbased on the reflection coefficients comprises dividing a magnitude ofthe measurement signal for the reflected signal by a correspondingmagnitude of the measurement signal for the forward signal.
 15. Themethod of claim 9, wherein detecting interference in one or morefrequency bins based on the interference detection metrics comprisescomparing the interference detection metrics for one or more frequencybins to the error threshold.
 16. The method of claim 9, whereincomputing interference detection metrics for one or more of thefrequency bins comprises computing measurement errors for each of thefrequency bins as a function of its corresponding reflectioncoefficient, and wherein the measurement errors comprise theinterference detection metrics.
 17. The method of claim 16, whereindiscarding frequency bins comprises discarding frequency bins where themeasurement error satisfies an error threshold.
 18. The method of claim16, wherein computing measurement errors for each of the frequency binsas a function of its corresponding reflection coefficient comprises:computing a forward coupling factor for each frequency bin as a functionof the measurement signal for the forward signal and a reference signal;and computing the measurement error as a function of the differencebetween the forward coupling factor and a reference value.
 19. Themethod of claim 16, wherein computing measurement errors for each of thefrequency bins as a function of its corresponding reflection coefficientcomprises computing the measurement error based on a difference betweenthe computed reflection coefficient and an expected reflectioncoefficient.